1. Field of the Invention
The present invention concerns a magnetic actuator embodied by a magnetic circuit formed of a fixed part and a mobile part which do not touch each other and which form between them an airgap into which a separator wall is inserted.
The magnetic actuator is more particularly a magnetic bearing and the wall is that of a confinement chamber, for example.
2. Description of the Prior Art
Specific applications of bearings require the use of a confinement chamber around one of the two relatively moving bodies. Such applications involve a high pressure, for example, a high temperature or a primary or hard vacuum. Other examples are corrosive, toxic or radioactive environments. It is generally not appropriate to place all of the bearing in the confinement chamber. This type of application therefore requires a perfect seal between the two parts of the actuator and therefore makes the use of static seals, which are necessarily rotary seals, between the mobile body and the other body difficult.
A magnetic actuator is then a beneficial solution since it enables a contactless coupling between the different bodies.
In a manner that is known in itself, for example from U.S. Pat. No. 4,918,345, a magnetic bearing is used for magnetically active centering about at least one centering axis of a body mobile relative to another body. The two bodies include ferromagnetic portions with no contact between them and the mobile body is said to be "suspended". Magnetically active centering implies the use of a winding in which flows a variable current adapted to generate a variable magnetic flux. A bearing of this kind generally further embodies permanent magnets that can in some configurations procure passive centering about one or more other axes. Magnetic bearings are frequently used in the case of rotary bodies but can also be used in the case of linear motion.
However, the walls of the confinement chamber extend into the airgap between the fixed part and the mobile part; these walls, which may be thick, must in principle be nonmagnetic so as not to create any magnetic short-circuit and therefore constitute a spurious airgap within the airgap of the magnetic actuator. This spurious airgap is even more of a problem if it is necessary to allow for differential expansion of the mechanical components due to temperature and/or compression, and therefore to provide an additional airgap. Finally, sufficient relative movement must be preserved between the fixed and mobile parts to guarantee the absence of contact.
This increase in the size of the airgap compared to a situation in which there is no wall degrades the performance of the magnetic actuator or increases the size of the components, the size of the windings, the excitation current, etc, if the same performance is to be maintained. This increase in the "dimensions" of the magnetic actuator is a cost penalty and also constitutes a penalty in terms of the space occupied by the actuator. It can even become technically impossible to implement it.
To reduce the required airgap consideration could be given to interrupting the wall at the level of the airgap and sealing the wall to the fixed body. This seal would be very difficult to achieve, however. The thickness of the wall could instead be reduced at the airgap, for example by means of a groove into which one of the bodies, for example the fixed body, would penetrate. This would reduce the mechanical strength of the chamber, however, and the latter would no longer fulfill its function.
The present invention is aimed at remedying these drawbacks of the prior art by providing a magnetic actuator having a wall in its airgap and the configuration of which has an airgap that is increased only a little or not at all compared to an actuator with no wall, so retaining substantially the same performance, while providing a seal and mechanical strength equivalent to that obtained with an actuator having a wall in a wider airgap.